Certain types of position determination systems, such as wheel alignment systems, use a vision imaging system that employs optical sensing devices to determine the positions of various target devices. A wheel alignment system of this type is capable of obtaining alignment information about a vehicle, such as toe, camber, ride height, toe curve, tilt angle, and the angular relationship of the vehicle's body relative to the vehicle's wheels.
The wheels of a motor vehicle may be aligned in a number of ways. For example, an operator or an alignment technician can use a vision imaging system, such as a computer-aided, three-dimensional (3D) machine vision that employs image sensing devices, such as cameras, to determine the positions of various target devices. Although such vision imaging systems are typically used for alignment purposes, these systems can also be used to obtain other positional and angular orientation information about a motor vehicle. Examples of such apparatus and methods are disclosed in U.S. Pat. No. 5,724,743, entitled “Method and Apparatus for Determining the Alignment of Motor Vehicle Wheels,” issued to Jackson, et al. on Mar. 10, 1998, U.S. Pat. No. 5,535,522, entitled “Method and Apparatus for Determining the Alignment of Motor Vehicle Wheels,” issued to Jackson, et al. on Jul. 16, 1996, and U.S. Pat. No. 6,526,665, entitled “Glint-Resistant Position Determination System, issued to Jackson on Mar. 4, 2003, each incorporated herein by reference.
A computer is often used in conjunction with such vision imaging systems to calculate the orientation of the target device by identifying certain geometric characteristics on the target device. The computer takes perspective measurements and compares these measurements with the true image previously pre-programmed into the memory of the computer.
An example of a typical target device that can be used in a wheel alignment system is illustrated in FIG. 1. Target device 64 consists of a flat plate with a pattern of two or more differently sized circles 62, 63 marked in a pre-determined format thereon. Although a specific pattern is shown, a large number of different patterns can be used on the target device 64. In practice, a mathematical representation, or data corresponding to a true image (i.e. an image taken by viewing the target device perpendicularly to its primary plane) and the dimensions of the target device are preprogrammed into the memory of a computer so that, during the alignment process, the computer has a reference image to which the viewed perspective images of the target devices can be compared.
The computer calculates the orientation of the target device 64 by identifying certain geometric characteristics on the target device. The computer takes perspective measurements and compares these measurements with the true image previously pre-programmed into the memory of the computer.
The computer could, for example, calculate the center of each of the circles 62a, 62b by means of centroiding. This is a method commonly used by image analysis computers to determine the positioning of the center point or centerline of an object. Once the center points of the two circles 62a, 62b have been determined, the distance between the two can be measured. This process is then repeated for other circles in the pattern on the target device 64. These distances can then be compared to the true distances (i.e. non-perspective distances) between the respective centers. Similarly, the angle to the horizontal (or vertical) of the line joining the two centers can be determined. A calculation can then be made of the orientation of the target device 64.
Other methods of calculation can be used to determine the orientation of the target device 64. For example, the camera could sight onto only one of the circles, for example the circle 63, and by using the perspective image thereof (the distorted ellipse), calculate the orientation of that circle and, therefore, the orientation of the target device 64, as more fully described in U.S. Pat. Nos. 5,535,522 and 5,724,743.
Existing targets commonly contain retro-reflective sheets sandwiched between front and back sheets of glass for protection from humidity and chemical contamination. The retro-reflective sheets generally comprise a rather fragile material. Neither glass layer adheres to the retro-reflective sheet. The combination is glued at its edges to a housing using hard epoxy glue to provide a seal and permanent attachment to the housing.
Wheel alignment systems are used predominantly in establishments such as automobile service centers, tire dealer shops, garages, repair shops, and the like. Such environments can subject the systems to a wide range of temperature, e.g., in the order of zero degrees F. to one hundred thirty degrees F. System targets additionally are often exposed to extremes of humidity and various harmful chemical products. Rough handling of the targets when being positioned on or removed from wheels is commonplace. Such handling of the target may cause the glue bond to loosen and allow humidity to wrinkle the retro-reflective sheet that is positioned between the two layers of glass. Resulting distortion can significantly affect the accuracy of the positioning system. Moreover, glass coverings of existing targets, as well as fragile retro-reflective material, can easily be broken if mishandled or dropped, rendering the targets unusable. Impact can be transferred through perimeter epoxy glue, imparting stresses to the glass elements with consequent breakage.
The need remains for alignment retro-reflective targets that are structurally stable over wide temperature ranges, that are protected from humidity and chemical contamination, and are better protected from breakage.